Can Ends and Closures
Ends for two and three-piece cans are usually stamped from scrolled sheets, i.e.sheets sheared in a pattern which minimises scrap loss by allowing staggered rows of circular ends to be stamped. The sheets are then inspected for surface appearance, correct gauge and absence of pinholes. High speed presses for making can ends stamp the end into the required profile. Leaving the stamp press, the ends are fed into a curling unit and then to a machine which applies a sealing compound around the inside of the curled edges; this ensures an hermetic seal when the closure is seamed on the can body. The ends are cured for 2-3 minutes. Bottom ends meet three-piece can bodies in can-making plants; top ends meet both two-piece and three-piece cans at the end of the filling line.
Ecology easy-open ends [“Stay-on-tab”] are widely made in aluminium for beverage cans.For such ends, the limits of the aperture are scored in the metal and a tab is riveted in place so that on lifting the tab, the aperture is ruptured inwards allowing access to the contents.The tab and aperture remain attached to the end. Different aperture shapes are available, the most common being oval.Increasingly, a “Gulper” or large opening end [LOE] is being used as this allows the product to be dispensed more rapidly from the can. Manufacturing speeds continue to rise substantially with multi-die shell and conversion presses predominating.
A number of beverage tinplate easy-open ends have been developed though not widely commercialised; with a harder, springy material like steel, the scoring operation is even more critical since the scoring must be deep enough to permit detachment by pulling, yet not so severe as to introduce a risk of leakage. Furthermore, there must be no damage to fingers from sharp edges.One design developed in Australia has a press-button can end for beer and soft drinks cans.
There are two circular press tabs of different sizes.These tabs are integrally hinged to the end and cannot be detached, thus they neither fall into the can contents nor add to litter problems.The end is opened by first depressing the smaller tab to release gas pressure and then depressing the larger tab to allow the contents to be poured. In a modification the smaller tab is situated inside the larger opening; this obviates the risk of contents spurting out when the buttons are pressed in.
These ends are manufactured by subjecting a normal end to a series of stamping operations, using transfer presses to form the tabs and apertures, applying a sealant around the internal cut edges to prevent corrosion and to provide an hermetic seal and coating the external cut edges with a repair lacquer. Tolerances for the die stages are said to be less critical than in the case of ring-pull openings and only low press tonnages are needed.
Aluminium and tinplate full aperture easy-open ends have been developed for both dry and processed foodstuffs.Normally, the full-aperture end is operated by a riveted ring-pull, similar to that used for beverage containers but detaching with the centre panel of the end. The end is scored circumferentially close to the end seam, so that the maximum area of the end is open when the panel is removed. Steel provides better performance for a given end thickness and is therefore more common where the food processing produces high internal can pressures.Harder steels have been the trend with much development around the industry currently focused toward using double-reduced steel.
This latter material is already common for classic [non-easy open] ends. With steel in particular, careful attention must be paid to all stages of the manufacturing process if a high level of end consistency is to be achieved.The achievement of acceptably low opening forces requires similar attention to product design.
Some aluminium full aperture ends have folds along one or both of the score edges as a safety feature.These are generally manufactured by collapsing a stepped shell profile in the conversion press during end manufacture.
Certain meat and fish packs have key opening.In the case of the meat packs, normally in rectangular cans, a narrow strip is scored into the can wall.This is removed from the body by inserting a tag from the strip into a metal “key” and winding off a metal strip, thus releasing one end of the container. In the case of shallow drawn containers for fish, one end is scored around the perimeter and the entire end is wound off using a detachable “key”.
In addition to ends for cans, tinplate is used to make bottle closures e.g. crown corks with characteristic corrugated rims and cork or plastic pad as sealant, ensuring a tight hermetic seal when applied to a bottle. Sheets of tinplate decorated with the bottle top pattern, are coated with wax and then passed to automatic presses which produce the bottletops from a series of dies.
A blanking punch cuts out the cap and the drawing punch passes through to form the crown. After blanking and forming, the closures are lined with composition cork, or moulded polythene reseals, or flowed-in plastic liners.A wide range of other glassware closures is produced in pre-decorated tinplate.Pre-formed screw caps are produced on special presses from decorated and lacquered tinplate and contain a plastic liner to ensure an hermetic seal. “Twistoff” closures are also made from tinplate stamped from sheet.